dnd: If needed, destroy _dragActor in _cancelDrag

In the case where the original actor is destroyed, we don't
show a snap back animation, so we need to destroy drag actor
as we would do in _onAnimationComplete.

https://bugzilla.gnome.org/show_bug.cgi?id=640781

js/ui/dnd.js

@@ -507,6 +507,9 @@ _Draggable.prototype = {
             if (!this._buttonDown)
                 this._dragComplete();
             this.emit('drag-end', eventTime, false);
+            if (!this._dragOrigParent)
+                this._dragActor.destroy();
+
             return;
         }